In recent years, as requirement of weight reduction and high output for an automobile are more highly required, a high stress design is directed also in a valve spring, a suspension spring or the like used for an engine, a suspension or the like. Therefore, for these springs, those which are excellent in fatigue resistance properties and setting resistance properties are strongly desired to cope with increase in a load stress. In particular, with respect to a valve spring, requirement for increasing fatigue strength is very strong, and even SWOSC-V (JIS G 3566), which is regarded to be excellent in fatigue strength among conventional steels, is becoming hard to cope with.
In a wire rod for a spring wherein high fatigue strength is required, it is necessary to reduce nonmetallic inclusions which become a start point of breakage present in the wire rod as much as possible. From such a viewpoint, with respect to the steel used for such usage as described above, it is common that high cleanliness steel wherein presence of the nonmetallic inclusions described above is decreased as much as possible is used. Further, because the risk of wire breakage and fatigue breakage due to nonmetallic inclusions increases as high strengthening of material is aimed at, the requirement for reduction and miniaturization of the nonmetallic inclusions which become its main cause has become more severe.
From the viewpoint of aiming at reduction and miniaturization of hard nonmetallic inclusions in steel, a variety of technologies have been proposed so far. For example, in the Non-patent Document 1, it is described that inclusions are refined in rolling by maintaining the inclusions at glass matter and that the inclusions are present in the CaO—Al2O3—SiO2 based component which is the composition wherein glass is stable. Also, it is proposed that lowering of the melting point of inclusions is effective in order to promote deformation of the glass portion (the Patent Document 1, for example).
Also, in the Patent Document 2, it is shown that a spring steel excellent in fatigue properties can be obtained by properly adjusting the chemical componential composition of steel while controlling quantity of Ca, Mg, (La+Ce) to a proper range, and making composition ratio of the average composition of non-metallic inclusions in steel (composition ratio of SiO2, MnO, Al2O3, MgO, and CaO) a proper range.
On the other hand, in the Patent Document 3, a wire rod for a high strength spring is proposed wherein excellent “setting properties” are exerted by controlling the fundamental components of C, Si, Mn, Cr, or the like, containing one kind or more out of Ca, Mg, Ba, Sr by the range of 0.0005-0.005%, and making the size of non-metallic inclusions 20 μm or below, and etc.
In a variety of conventional technologies proposed so far, aiming of refinement by controlling the composition of inclusions to a low melting point region is centralized. For example, in CaO—Al2O3—SiO2 three-component based inclusions, it is known that a low melting point region is present in a composition area of three components in the three component system phase diagram which is generally known, however, in a composition where any of the components becomes high, the melting point becomes high and the fatigue strength of the wire rod lowers. Such tendency is similar also in the case of MgO—Al2O3—SiO2 three-component based inclusions.
In a variety of technologies described above, the direction for improving properties such as fatigue properties is shown. However, in the heating time and temperature during hot working, the perfect glass state cannot necessarily be kept only by controlling the composition to that as shown in the Non-patent Document 1 for example, and crystals may possibly be formed. Also, in order to cope with the needs of further strengthening of fatigue strength of steel in recent years, it is necessary to further promote deformation of the glass portion as well.
Further, with high strengthening of steel, content of Si in steel is increased, degree of difficulty of pin-point control aiming the target composition in conventionally known CaO—Al2O3—SiO2 system is in the tendency of becoming high, and as shown in the Patent Document 4 for example, a sophisticated control such as controlling not only totally but also the dissolved component has become necessary.
Also, as a technology for making inclusions harmless (against fatigue), a technology of controlling the composition of inclusions is disclosed. For example, in the Non-patent Document 1, it has been disclosed that, in valve spring steel, if controlled to CaO—Al2O3—SiO2 three-component based inclusions whose melting point is lower than approximately 1,400-1,500° C., they do not become the start point of fatigue failure and fatigue properties improve.
Furthermore, in the Patent Document 5, it is shown that cleanliness steel excellent in cold workability and fatigue properties can be obtained by that the average composition of non-metallic inclusions whose length (l) and width (d) ratio is l/d≦5 in L-section of rolled steel contains SiO2: 20-60%, MnO: 10-80%, and either one or both of CaO: 50% or below and MgO: 15% or below.
In the Patent Document 6, it is shown that cleanliness steel excellent in cold workability and fatigue properties can be obtained by that the average composition of non-metallic inclusions whose length (l) and width (d) ratio is l/d≦5 in L-section of rolled steel is made to comprise SiO2: 35-75%, Al2O3: 30% or below, CaO: 50% or below, MgO: 25% or below.
In the Patent Document 2, it is disclosed that, fatigue strength is improved by controlling SiO2: 25-75%, Al2O3: 35% or below, either one or both of CaO: 50% or below and MgO: 40% or below, and MnO: 60% or below to be contained in inclusions.
In the Patent Document 1, it is disclosed that, fatigue strength is improved by controlling the melting point of the inclusions whose melting point is highest to 1,500° C. or below.
Also, with respect to the technology using a special component, there is one shown in the Patent Document 7 wherein inclusions are controlled to Li2O composition, and one shown in the Patent Document 3 wherein Ba, Sr, Ca, Mg are contained in steel.
In these conventional technologies, it is described that the composition is controlled to one wherein vitrification is easy in order to promote deformation of inclusions in hot rolling, and that inclusions are controlled to of low melting point composition in order to further promote deformation. Also, with respect to a specific inclusions composition, a SiO2-based composite oxide system wherein glass is stable is shown.
However, it is not possible to cope with the needs of further strengthening of fatigue strength properties from now only by the conventional methods described above. Also, even if further lowering of the melting point is tried on a system of SiO2—Al2O3—CaO—MgO—MnO or the like on which many reports have been conventionally given aiming to make inclusions of lower melting point in order to further promote deformation, the situation has already reached wherein further improvement is difficult.
Further, in the Patent Document 3 described above, utilization of Ba, Ca, Mg, Sr, or the like is cited, however, only their effect of lowering the melting point is focused and difference of each composition and the effect of compositing combination are not utilized, which results in the technology wherein the fatigue strength capable of meeting current high requirement cannot be realized.
Also, it is difficult to obtain the low melting point inclusions with those containing much Al2O3 among non-metallic inclusions, therefore it is common that the steel for obtaining such wire rod adopts so-called “Si-killed steel” deoxidizing using Si instead of Al-killed steel.    Non-patent Document 1: “182nd and 183rd Nishiyama Memorial Technical Lecture”, edited by The Iron and Steel Institute of Japan, pp. 131-134.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H5-320827    Patent Document 2: Japanese Unexamined Patent Application Publication No. S63-140068    Patent Document 3: Japanese Unexamined Patent Application Publication No. S63-227748    Patent Document 4: Japanese Unexamined Patent Application Publication No. H9-310145    Patent Document 5: Japanese Unexamined Patent Application Publication No. S62-99436    Patent Document 6: Japanese Unexamined Patent Application Publication No. S62-99437    Patent Document 7: Japanese Unexamined Patent Application Publication No. 2005-29888